The relevant changeset is 1570e3855ce8. Here is a traceback from a random buildbot:

======================================================================
ERROR: test_port_parameter_types (test.test_asyncio.test_base_events.BaseEventTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/dje/cpython-buildarea/3.x.edelsohn-debian-z/build/Lib/test/test_asyncio/test_base_events.py", line 143, in test_port_parameter_types
    base_events._ipaddr_info('1.2.3.4', b'1', INET, STREAM, TCP))
BytesWarning: Comparison between bytes and string

http://buildbot.python.org/all/builders/s390x%20Debian%203.x/builds/1100/steps/test/logs/stdio

I really can't figure out what's going on. I can't reproduce this, and the exception doesn't make any sense :(

Alright, I think I know what's going on here -- this seems to be a bug of functools.lrucache.  Serhiy, would you be able to look into this?

Testcase:

test.py:

    import functools

    @functools.lru_cache()
    def foo(a):
        return

    foo('')
    foo(b'')

$ python3.5 -bb test.py
Traceback (most recent call last):
  File "test.py", line 8, in <module>
    foo(b'')
BytesWarning: Comparison between bytes and string

The workaround is using lru_cache(typed=True).

> The workaround is using lru_cache(typed=True).

Yes, seems to be working! Thanks.

Berker, it should be fixed now in https://hg.python.org/cpython/rev/bdfb15aa3ac5 (sorry, I forgot to reference the issue in the commit message)

Yes, buildbots look happy now. Thanks!

On Python 3, comparison between str and bytes raises a BytesWarning exception if python3 is started with -bb. But sometimes, you really want to compare str with bytes. For example, os.get_exec_path() changes temporarely the BytesWarning warning for that!

    # {b'PATH': ...}.get('PATH') and {'PATH': ...}.get(b'PATH') emit a
    # BytesWarning when using python -b or python -bb: ignore the warning
    with warnings.catch_warnings():
        warnings.simplefilter("ignore", BytesWarning)
        ...